The present invention relates broadly to the detection of helicopters, and more particularly to a pulse doppler radar system which detects and identifies an operational helicopter and derives the unambiguous range thereof utilizing the specular flash energy of the doppler shift frequency signals reflected from the rotating blades of the helicopter.
Airborne surveillance pulse doppler radars, in some cases, provide advance warning of enemy and unidentified aircraft flying within their scanning purview. It is well known that certain types of helicopters may be equipped with offensive weaponry to become a viable military threat. To the military, then, the detection, identification and tracking of helicopter targets by the advanced warning airborne radars are of paramount importance. Allowing helicopter targets to avoid detection of the airbore radars may result in the unbalancing of military position.
Most airborne pulse doppler radars include signal filtering circuitry to cancel or reject doppler shift frequency signals derived from the reflections of stationary and slow moving objects, generally known as radar clutter. The minimum detectable target velocity of these clutter cancelers is usually adjusted to include doppler shift frequency signals of slow moving surface vehicles, like automobiles, for example. Unfortunately, the velocity range of most hovering and slow moving helicopter targets falls below this minimum detectable target velocity. Consequently, their doppler shift frequency signals are also canceled in the airborne radars, permitting these helicopter targets to go undetected. Now, it is always possible to reduce the minimum detectable target velocity of airborne radars to detect the helicopter targets; however, this will also result in the additional detection of the moving surface vehicles. Since there exists, in general, a significantly larger number of moving surface vehicles than helicopter targets, the probability of distinguishing between the doppler shift frequency signals of a target helicopter and the moving surface vehicles with present airborne radars is extremely low. In fact, the detection of a helipcopter target under these conditions may be considered virtually impossible. Therefore, it becomes readily apparent that another approach must be considered for detecting these helicopter targets, something beyond that of utilizing the velocity of the moving craft itself.
In another aspect of aircraft target detection, most pulse doppler airborne radar sets function with medium to high pulse repetition frequency (PRF) transmissions, on the order of 25 KHz, for example. As a result, the range indication of a detected target aircraft is ambiguous in that there are a plurality of range cells at which the detected target will satisfy the timing of the received signal return of a particular PRF of the airborne radar. To resolve the ambiguities in range, some airborne radars transmit radar signals at a plurality of PRF's and correlate the received signals from the detected target. In the correlation process, only the unambiguous range of the detected target will remain fixed while the ambiguous ranges will vary in proportion to the particular PRF being transmitted. If helicopters are also included as potential moving aircraft targets of the airborne radar, considerations must also be given to resolving the ambiguities in the range thereof. Since the detection of helicopters may involve a characteristic of the craft other than its body or skin velocity, a simple variation in PRF to resolve ambiguities in the range of the detected helicopter may not be sufficient.
Still another aspect of aircraft target detection is the ability of most airborne radars to discriminate between detected moving targets within their radar scanning purview. Generally, it is not until one detected target aircraft is distinguished from another can the ambiguous range thereof be unambiguously resolved. Known airborne pulse doppler radar sets employ a plurality of doppler shift frequency filter banks to distinguish between a variety of moving target aircraft. In these systems, it is basically assumed that each moving target aircraft has a unique velocity vector (i.e. speed and direction) which separates it from the others. Thus, each detected target aircraft is resolved uniquely into corresponding doppler frequency bands or cells. And thereafter, each identified target may be processed individually from its correspondingly associated doppler frequency band. Since it is likewise possible to have more than one moving helicopter target within the scan of the pulse doppler radar set, it may be additionally necessary, in any helicopter detection system, to distinguish between multiple helicopter targets in some cases. However, pulse doppler radar detection of helicopters may involve characteristics endemic to only helicopters which may yield doppler shift frequencies which are inapplicable to the known teachings of partitioning aircraft targets into doppler frequency bands corresponding to their velocity vectors. Here again, for helicopter detection with pulse doppler radars, another way of distinguishing one craft from another may be required under certain situations or for all cases.
Representative of pulse doppler radar sets is the disclosed subject matter found in the following U.S. Patents all assigned to the same assignee as of the present application:
(1) U.S. Pat. No. 4,079,376 entitled "Target Detection System in a Medium PRF Pulse Doppler Search/Track Radar Receiver"; issued to John C. Kirk; filed on Mar. 9, 1976 and issued Mar. 14, 1978;
(2) U.S. Pat. No. 4,093,948 entitled "Target Detection in a Medium PRF Pulse Doppler Radar"; issued to H. Long III; filed on June 8, 1976 and issued on June 6, 1978;
(3) U.S. Pat. No. 4,095,222 entitled "Post Detection STC in a Medium PRF Pulse Doppler Radar"; issued to David H. Mooney, Jr.; filed on May 27, 1976 and issued on June 13, 1978; and
(4) U.S. Pat. No. 4,137,532 entitled "VIP Doppler Filter Bank Processorfor Pulse Doppler Radar"; issued to John W. Taylor, Jr. et al.; Filed on Apr. 29, 1977 and issued June 30, 1979.
All the patents set forth above are provided as a reference herein for a more detailed description of known pulse doppler radar sets.
Moreover, in a helicopter detection radar system, just like in any other radar aircraft detection system, the ability to distinguish real reflection signals of target helicopters from false pulse interference signals from enemy jammers or even friendly radar, for example, should be included. Without protection against pulse interference, the detection system may be rendered ineffective and caused to respond to false detection of helicopter targets. Thus, the viability of such a detection system would be deemed inadequate for the purposes for which it was designated. Therefore, in any helicopter detection radar system consideration must be additionally given to the handling of strong, high amplitude, impulse radar interference.